How to declare a specific key for has_many association with :through option?

Question

I have an association:
an author has many books; and a book has many authors;
I need to use :through option(through a table named 'relations',has two column named 'left_id' (used as author_id) and 'right_id' (used ad book_id);

---

class Relation < ActiveRecord::Base
  belongs_to :books
  belongs_to :authors
end

class Author < ActiveRecord::Base
  has_many :relations, :foreign_key => 'left_id'
  has_many :books, :through => :relations
end

---

In the console:

---

> author = Author.new
> author.books 
#  => Error: no such column: relations.book_id

---

So, how could I specific the 'book_id' to 'right_id'?(Is there some option like 'foreign_key'?)

Answer 1

I don't know about foreign_id and can't find anything on google. How about this?

has_many :relations, :local_key => 'left_id', :foreign_key => 'right_id'

Answer 2

You should use a foreign_key also in the Relation model, so the belongs_to has also foreign_key. More specifically this is what you need:

class Relation < ActiveRecord::Base
  belongs_to :book, :foreign_key => :left_id
  belongs_to :author, :foreign_key => :right_id
end

and the other models should be:

class Book < ActiveRecord::Base
  has_many :relations, :foreign_key => :left_id
  has_many :authors, :through => :relations
end


class Author < ActiveRecord::Base
  has_many :relations, :foreign_key => :right_id
  has_many :books, :through => :relations
end